Elastic Constants and Coefficients of Thermal Expansion of Laves Phase Cr2X (X=Hf, Nb, Ta, Zr) Alloys

Abstract

ABSTRACTTo examine the effect of thermal expansion mismatch on cracking in two-phase Cr-Cr2X (X=Hf, Nb, Ta, Zr) alloys, we calculate the elastic constants and associated elastic anharmonicity by the local-density-functional approach from which the coefficients of thermal expansion (CTEs) of Cr and Cr2X are determined. The calculation shows that the CTE of Cr at high temperatures is notably larger than those of Cr2X. If the difference in CTE between Cr and Cr 2X is a primary source of crack initiation, our results fail to explain the experimental observation that, among Cr-Cr2X alloy systems, the ingot cracking occurs mainly in Cr-Cr2Nb. We suggest that for the cracking to occur, thermal mismatch stresses are retained by a hard and supersaturated Cr matrix (e.g., due to the relatively high solubility of Nb in Cr in the case of Cr-Cr2Nb). On the other hand, a soft Cr matrix can accommodate the thermal misfit dislocations plastically even when the CTE difference between Cr and Cr2X is large (e.g., in the case of Cr-Cr2Zr).